The present invention relates to the measurement of the electrical characteristics of semiconductor product wafers and, in particular, the contactless measurement of junction leakage for different type junctions.
It is often desirable to make P-N junction leakage measurements on product wafers that contain many P-N junctions. A simple qualitative leakage measurement may be obtained by examining the median leakage behavior of all the P-N junctions on single chip. This may be done by exciting P-N junctions with light and examining the resultant photovoltaic decay of the junctions with a large capacitive sensor that is the approximate size of the product chip. Such a method works well when one particular type of P-N junction tends to dominate the general area of the chip. Examples would be a CMOS design, where the chip area was dominated by N wells or P wells, or for a bipolar design, where the chip area was dominated by subcollectors.
However, in many designs of interest, a particular type junction does not dominate, yet a leakage measurement for that type junction is desired. For example, in a CMOS design, it is also of interest to be able to examine leakage of the source-drain junctions that are located in the same N wells or P wells mentioned above. However, the photovoltaic signals from the much smaller source-drain junctions will tend to be much smaller (e.g., one tenth or less) of the photovoltaic signals from the N and/or P wells. A method for separating the photovoltaic signals from the differing types of junctions is needed.